Hitherto, it is known that, for example as shown in FIG. 1, in a pressure control valve of a conventional type: a relief poppet c functions to permit and prevent communication between a pressure supply port "a" and a relief port b; a sleeve-like piston poppet e is slidably mounted in the relief poppet c, and extends to pass through the same c so as to permit communication between the pressure supply port "a" and a back-pressure chamber d; a pilot poppet g functions to open and close a passage between the back-pressure chamber d and a low-pressure relief port f, a ratio of which passage to the piston poppet c in cross section, i.e., a cross-sectional area A.sub.P of the passage to that A.sub.R of the piston poppet e is a value of A.sub.P /A.sub.R which is so determined as to be greater than or equal to 1.1, whereby the pressure control valve is improved in transient properties of operation (See Japanese Patent Publication No. Sho 62-29665). In the conventional pressure control valve having the above construction, a pressure of pressurized oil existing in the relief port f is low when the pilot poppet g begins to open. In contrast with this, a pressure of pressurized oil existing in the relief port b is very high when the relief poppet c begins to open. Consequently, a difference in pressure between the pressurized oil in the low-pressure relief port f and that in the relief port b is very large in the conventional pressure control valve (See FIG. 2).
Namely, in operation, the transient properties of the conventional pressure control valve depends on relief-flow properties of the pilot poppet g instead of relief-flow properties of the relief poppet c until a pressure of the pressurized oil existing in the pressure supply port "a" increases to an eventual value at which the relief poppet c begins to open, which pressure increases first to a value at which the pilot poppet g begins to open. Consequently, in the above transient period of time in operation, the pressurized oil flows to a drain side of the pressure control valve. This flow of the pressurized oil is not advantageous from the standpoint of energy saving.
In other words, in operation of a hydraulic system, since control of a hydraulic actuator of the system depends on relief-flow properties of the relief poppet c of the pressure control valve in the system, such control of the actuator depends on relief-flow properties of the pilot poppet g of the pressure control valve until the relief poppet c of the pressure control valve is opened. Consequently, relief-flow properties of the conventional pressure control valve are shown in FIG. 2.
As is clear from the relief-flow properties shown in FIG. 2, in the conventional pressure control valve having the above construction, a difference in pressure between: a pressure with which the pilot poppet g is opened; and a pressure with which the relief poppet c is opened becomes large in the transient period of operation. In addition, during such transient period of operation, motion of the pilot poppet g in the pressure control valve becomes unstable and tends to cause chattering.